Saint Syrus' history can be found in the De laudibus Papiae (In the Praise of Pavia). In the book, the story tells that Saint Syrus is the boy with the five loaves that appears in the Gospels. Hippolyte Delehaye wrote :
"To have lived amongst the Saviour's immediate following was...honorable...and accordingly old patrons of churches were identified with certain persons in the gospels or who were supposed to have had some part of Christ's life on earth."
The story continues that Saint Syrus followed Saint Peter to Rome before being sent to the Po Valley to convert the many peoples living there and in the cities of northern Italy.
A second tradition that has it's roots in the 8th century holds that Saint Syrus was a disciple of Saint Hermagoras, the founder of the diocese of Aquileia and a disciple of Saint Mark the Evangelist. Saint Syrus was sent there by him and was reported to be the first Bishop of Pavia. He worked with Juventius of Pavia to push back against the growing wave of Arianism in the area.